1. Field of the Invention
The present invention relates to vacuum insulator panels, constructions using vacuum insulator panels, and their manufacture.
2. Description of Related Art
Thermal insulator casings are used in the construction of various types of devices and systems, such as refrigerators, freezers, show-cases, etc. The most widely used thermal insulation material is polyurethane foam. A conventional thermal insulator casing has a metal outer casing, a plastic inner casing and polyurethane foam between the outer and inner casings. Polyurethane foam is a reasonably good insulator. However, even better insulation can be achieved using a vacuum insulator panels instead of polyurethane foam between the outer and inner casings.
A vacuum insulator casing comprising a vacuum insulator panel is disclosed in U.S. Pat. No. 4,681,788. The vacuum insulator panel is formed by a thin plate. The panel is wrapped by a sealed shell which includes a multilayer film having thin metal foil laminas to prevent gas leakage. .sub.13 An inner space defined by the shell is filled with mixture of precipitated silica and fly ash as a filer. Air in the shell is evacuated to create a vacuum inside the shell. Then, the film is air tightly sealed by any suitable means, e.g., hot melting or adhesive bonding. These vacuum insulator panels are fixed at an outer casing or an inner casing. Any additional space between the outer casing or the inner casing and the vacuum insulator panels is filled with foamed in place polyurethane foam. A vacuum insulator casing of this type provides a better thermal insulating performance than can be achieved using a conventional polyurethane foam insulator casing of comparable thickness. However, at the polyurethane foam portion between the inner casing and the outer casing, the thermal insulating performance is the same as that of a conventional insulator casing.
An alternative embodiment of a vacuum insulation casing is also shown in U.S. Pat. No. 4,681,788. A vacuum insulator panel is sandwiched between outer and inner casings. This vacuum insulator casing has a high degree of thermal insulation. However, when vacuum insulator panels are used to construct a refrigerator or the like, it is necessary to make corner sections to fit the space between the outer and inner casings. Corner sections of a vacuum insulator panels must be harder than straight sections because of the additional forces at corner sections. A hard vacuum insulator panel can be made by using a high density filler in the shell. However, using a high density of filler decreases the space available for vacuum. Thus, using a high density of filler decreases the thermal insulating performance of the panel. Another way to achieve the needed strength is to use a harder shell. However, if a harder shell is used, the shell and the vacuum insulator panel becomes thick. A thick shell usually has a high thermal conductivity, so its thermal insulating performance is decreased. Therefore, it is difficult to fill up all of the space in the insulator casing, i.e. the space between the outer casing and the inner casing, using vacuum insulator panels.
Another problem associated with the use of convention vacuum insulator panels has to do with their disposal. Proper disposing of a vacuum insulator casing requires that it first be analyzed to determine its constituent parts. In conventional vacuum casings, a vacuum insulator panel is firmly fixed with the outer or inner casing, or buried within polyurethane foam in order to improve it's thermal insulating performance. Therefore, it is difficult to analyze.
Another known vacuum insulator panel uses hard polyurethane foam having a continuous foam as a filler instead of using a mixture of precipitated silica and fly ash. The continuous foam is made by mixing foamed in place material with foam breaker when it is foaming. However, the surface of hard polyurethane foam tends to remain as a individual foams. It is impossible to evacuate the air from the individual foams. Thus, the individual foams on the surface of the hard polyurethane reduce the thermal insulating performance of the vacuum insulator panel.